It is therefore customary to subject the plated-through holes of printed circuit boards to a visual examination, preferably under a microscope, and along a diametrical cross-section through a row of holes. Since this is a destructive process, it is performed on a fragment of a circuit board, a so-called coupon. The coupon may be an appendage produced and plated-through together with the circuit board and subsequently broken off from the board for examination. Alternatively, the coupon may be cut out from a board, which is thereby destroyed. In that case, the coupon will be taken to be representative of a larger series of identical boards which have been subjected to the plating-through operation under identical conditions.
In either case, the coupon is so made that it has a row of holes to be examined located at a short distance from one longitudinal edge of the coupon. The row of holes to be examined has a common diametrical plane which forms a potential inspection face of the coupon. To expose this inspection face for the purpose of visual examination, material is removed from the relevant edge of the coupon and inwards to a depth such that the resulting cross-sectional inspection plane exactly coincides with the position of the common diametrical plane of the holes. This removal of material usually takes place by grinding, preferably in a plurality of steps, and is preferably followed up by at least one polishing step to smooth the metal surfaces to be examined under the microscope.
It is essential that the grinding depth should be controlled with very high precision, e.g., within tolerance limits of .+-.0.05 mm. As a means of achieving this it is common practice to provide each coupon with two reference holes which are bored through the coupon adjacent its ends in positions such that the common diametrical plane of the reference holes is located at a predetermined accurate distance from the common diametrical plane of the row of holes to be examined. These reference holes may be used for the insertion of two hardened steel pins, referred to as positioning pins, by means of which the coupon may be held in correct position in the grinding and polishing steps. Preferably a plurality of coupons, e.g., six coupons, are mounted in a spaced side by side array on the same two positioning pins so as to form a set of coupons for joint handling in the examination procedure, the ends of the positioning pins extending beyond the outermost coupons of the set.
In a known method, a plurality of such sets of coupons are placed in respective mold cavities provided in the front face of a discshaped sample holder belonging to a grinding and polishing machine, in such a manner that the extending ends of the two positioning pins of each set of coupons extend beyond the circumference of the respective cavity and are supported against the front face of the disc. A curable plastic material, such as acrylic resin, is poured into the cavities to embed the coupons and anchor them with the positioning pins in firm contact with the front face of the disc, whereby the samples so formed have the plated-through holes of all their constituent coupons located at a precise distance in front of the front face of the disc. The sample holder is provided with axially-extending adjustable stop screws having hardened end portions, e.g., consisting of tungsten carbide, resistant to the action of a grinding face. By adjusting the stop screws such that their ends project to a predetermined distance from the front face of the disc, the large number of coupons comprised by the plurality of sets of coupons may simultaneously be ground down to the same cross-sectional level as related to the positioning pins. If more than one grinding step is to be performed, e.g., a rough grinding and a fine grinding operation, the stop screws must each time be re-adjusted to a smaller distance from the front face of the disc, and in the final grinding step this distance should be exactly equal to the radius of the positioning pins plus the distance between the diametrical plane of the reference holes and the diametrical plane of the row of holes to be examined. If the grinding steps are to be followed up by one or more polishing steps, the stop screws must again be re-adjusted in order to retract them so that they will not engage the polishing face.
While satisfactory results may be obtained with this known method if carried out with great skill and care, it is a disadvantage that the sample holder is unavailable for active use during the relatively long time interval, e.g., 1/2-1 hour, required for the curing of the plastic material used for embedding, considering that the actual processing time for grinding and polishing amounts to just a few minutes. In large scale procesing, it will therefore be necessary to have a large number of interchangeable sample holders available for use. It is also a disadvantage that a plurality of stop screws have to be adjusted and checked for accuracy before the commencement of each grinding step. Both of the factors mentioned tend to reduce the productivity of both the grinding machine and the operators.